The present invention generally concerns filter capacitors and more particularly, the organization of internal electrodes within a feed-through filter capacitor.
The present invention relates to the placement of internal electrodes within a multi-layer, feed-through filter capacitor made of a dielectric material such as a ceramic dielectric material. Wires carrying electrical signals to be filtered pass through one or more holes that are most commonly aligned to the axis of the disc. Capacitance between spaced-parallel plate regions is a function of their separation. Further, plate density cannot be particularly high in a multi-layer capacitor that relies on only a relatively thin ceramic layer to limit the breakdown voltage. It has heretofore been believed that metal plate regions of alternating polarity should be stacked along and transverse to an axis the hole. The metal plate regions are normally in a parallel relationship and partially overlap each other. The metal plate regions are parallel and overlapping so as to create capacitance along the elementary model of two parallel plate electrodes. The formula for the capacitance of the conventional parallel-plate ceramic capacitor is   Cap  =      kA    d  
where
Cap is the capacitance in farads,
k is the dielectric constant in farads per meter,
A is the area of electrode overlap in square meters, and
d is the distance of separation between plates in meters.
Although d would desirably be minimized for greatest capacitance, in high voltage capacitors, d cannot be indefinitely small or else the capacitor will be subject to failure from voltage breakdown of the insulating ceramic dielectric. For example, referring to FIGS. 1A and 1B, a known multi-layer, discoidal, feed-through ceramic filter capacitor 1 has a central hole, or bore, that is typically surfaced with first conductive metal 11, and external rim surface, or circumference, that is typically surfaced with a second conductive metal 12. Conductive metals 11 and 12 may be the same type of metal. The capacitor 1 is substantially made from multiple layers 13x of ceramic 13. Between the layers 13x are a number of ring-shaped first metallized areas, or plates, 14x that collectively form a first electrode 14, and a number of ring-shaped second metallized areas, or plates, 15x that collectively form a second electrode 15.
The external diameter D of the capacitor 1 is typically about 0.105 inch (xe2x80x9cin.xe2x80x9d) or 105 mils; the internal diameter d is about 35 mils; and the overall thickness T is about 65 mils. A typical ceramic dielectric will have a voltage rating of 100 volts per mil (0.001 in) thickness. For example, if the capacitor 1 is designed to have a breakdown voltage of about 1000 volts, an axial plate separation, that is, the ceramic dielectric thickness t in the axial direction between adjacent plates 14x, 15x must be about 10 mils.
Another aspect of high voltage ceramic capacitor design relates to the distance d1 of separation between any electrode plate 14x, 15x and respective external metal 11, 12 in the radial direction along the layers 13x. The radial plate separation d1 should be 50% greater than the plate separation in the axial direction transverse to the layers 13x. This is because a voltage breakdown is more likely to occur along the unavoidable imperfections of the seams 16 between layers 13x. Thus, the distance d1 should be about 15 mils, that is, 1.5xc3x9710 mils.
The internal design of the prior art feed-through multi-layer ceramic capacitor 1 with a 1000 volt rating shown in FIG. 1B has a thickness t of 10 mils layer-to-layer, and an end margin d1 to both electrodes 14, 15 of 15 mils. The capacitor 1 has top and bottom ceramic covers that are 7.5 mils thick, and therefore, the overall thickness of the ceramic 13 is about 65 mils. Given a desired separation of 10 mils, the number of active internal plates 14x, 15x is thus three of each polarity; and those plates have a total overlapping area A of 0.001099 sq.in., that is, ((0.0375 in)2xe2x88x92(0.0325 in)2)xcfx80.
Capacitors so constructed are of particular use to filter electrical signals upon the wires and leads of implanted cardiac pacemakers and cardiac defibrillators. These latter devices use high voltages, commonly about 750 volts. When the electrodes of a filter capacitor are subjected to high voltages, for example, on the order of hundreds and, with safety margins, even thousands of volts, the partially-overlapped metal plates 14x, 15x are subject to develop voltage breakdown paths. Such paths can occur between adjacent plates through the ceramic and/or to oppositely-charged regions of the outside surface of the capacitor where electrical connections are made.
Thus, there is a need for an improved laminated discoidal feed-through ceramic filter capacitor of substantially the same size that provides a greater capacitance while at the same time has a substantially higher breakdown voltage.
The present invention provides a multi-layer, feed-through filter capacitor that has a significantly higher voltage breakdown threshold than known capacitors of comparable size. Thus, the filter capacitor of the present invention is especially useful in applications where higher voltages may be expected and can be used in a wider range of more rigorous applications than known comparable capacitors. The feed-through filter capacitor of the present invention has a further advantage of being able to easily measure the capacitance of each of the filter and coupling capacitors within the feed-through filter capacitor.
According to the principles of the present invention and in accordance with one embodiment, the present invention provides a feed-through capacitor having layers of dielectric material. A feed-through filter capacitor has layers of dielectric material having at least one hole passing therethrough with first electrodes disposed on the dielectric material layers and extending in a first direction substantially perpendicular to a centerline of the hole. Second and third electrodes are disposed on layers of the dielectric material and also extend in the first direction. Any one of the first, second and third electrodes are non-overlapping with any of another of the first, second and third electrodes in a second direction substantially parallel to a centerline of the hole. The non-overlapping electrodes provide a feed-through filter capacitor having a higher voltage breakdown threshold than known capacitors of comparable size.
In one aspect of this invention, a first capacitor is formed substantially wholly by fringe-effect capacitance between the first and third electrodes, and a second capacitor is formed substantially wholly by fringe-effect capacitance between the second and third electrodes. First, second and third electrode contacts are electrically connected to respective first, second and third electrodes. The first capacitor is electrically connected between the first and third electrode contacts, and the second capacitor is electrically connected between the second and third electrode contacts. The first and the second capacitors are electrically connected in series between the first and the second electrode contacts. User accessibility to the third electrode permits the electrical characteristics of each of the first and second capacitors to be independently measured.
In another embodiment of the invention, a feed-through filter capacitor has layers of dielectric material having at least two holes passing therethrough with first electrodes disposed on layers of the dielectric material and extending in a first direction substantially perpendicular to a centerline of the hole. In addition, second, third and fourth electrodes are disposed on layers of the dielectric material and extend in the first direction. Any one of the first, second, third and fourth electrodes are non-overlapping with any of another of the first, second, third and fourth electrodes in a second direction substantially parallel to a centerline of one of the holes.
In one aspect of this invention, a first capacitor is formed substantially wholly by fringe-effect capacitance between the first and fourth electrodes; a second capacitor is formed substantially wholly by fringe-effect capacitance between the second and fourth electrodes; and a third capacitor is formed substantially wholly by fringe-effect capacitance between the fourth and third electrodes. First, second, third and fourth electrode contacts are electrically connected to the respective first, second, third and fourth electrodes. The first capacitor is electrically connected between the first and the fourth electrode contacts; the second capacitor is electrically connected between the second and the fourth electrode contacts; and the third capacitor is electrically connected between the fourth and the third electrode contacts. The first and the third capacitors are electrically connected in series between the first and the third electrode contacts, and the second and the third capacitors are electrically connected in series between the second and the third electrode contacts. Again, user accessibility to the fourth electrode permits the electrical characteristics of each of the first, second and third capacitors to be independently measured.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.